1. Field of the Invention
The present invention relates to the fabrication of photomasks, and, more particularly, to the fabrication of photomasks used to form microlenses, located above a light-receiving unit used for photoelectric conversion in an image sensing apparatus, such as a CCD sensor or a CMOS sensor.
2. Description of the Related Art
Conventionally, image sensing apparatuses contain a condensing microlens for each pixel, to increase the light collection efficiency of a light-receiving unit.
The microlenses are formed as follows: a photosensitive resin is formed into a pattern of separated spots, corresponding to individual pixels, by a photolithographic method, the resin pattern of separated spots is softened by heating, and a surface of the resin pattern is made spherical by surface tension.
With the refinement of pixels, the sensitivity of the image sensing apparatus is decreasing. Thus, it is desired to reduce spacing between the microlenses, to collect light entering between the microlenses. However, with methods that soften the resin pattern by heating, it is difficult to reduce the spacing between the microlenses. This is because some gap must be provided between the lenses to prevent contact between adjacent lenses when the resin is softened by heat treatment.
On the other hand, methods have been proposed that form microlenses by exposing a photosensitive resin placed under a photomask, and then developing the exposed photosensitive resin, in which the photomask is capable of controlling a transmitted quantity of exposure light by a dot pattern consisting of microdots (see, for example, Japanese Patent Laid-Open Nos. 2004-145319 and 2004-70087).